An illumination apparatus such as a flash light, for example, which illuminates a subject when the subject is photographed by a camera installed in an instrument such as a mobile phone or the like has been conventionally used. In such an illumination apparatus, it has been difficult to obtain white color illumination light by mixing light produced by light emitting diodes (hereinafter referred to as LEDs) of three primary colors, and therefore, a white light source in which a blue light emitting diode chip is sealed by a resin containing a phosphor such as yttrium aluminum garnet (hereinafter referred to as YAG) or the like has been used. However, because the YAG contained in the resin looks yellow and this yellow is considered to spoil design of an instrument, a semi-transparent member or member having low light transmission is provided in a window disposed in front of the white light source so that yellow color YAG does not become clearly visible. A technology to achieve white illumination light by adding a fluorescent material to a resinous mold of a blue light-type LED has also already been proposed (see Patent Document 1). In addition, a detailed review concerning selection of the added materials has also been made (see Patent Document 2).
However, use of the semi-transparent member mounted on the front window of the LED gives rise to a problem that light emitted from the LED is attenuated through diffusion, absorption and reflection or the like by the semi-transparent member. It is required to consider the attenuation rate of the light to be 50 to 80%, with the result that LED has very low illumination efficiency. Also, it is said to be ideal for an illumination apparatus that illumination width of light should be within ±30 degrees to light axis. However, in this apparatus, particles contained in the member for the window being semi-transparent scatter light emitted from the LED light source. As a result, even if a light focusing system such as a reflector, lens and so on is provided with the LED light source, light focusing effect cannot be fully achieved and illumination width tends to be wider than the ideal width. This is explained below, referring to FIG. 5 showing a conventional subject illumination apparatus.
The illumination apparatus includes an LED light source 30 disposed in a casing 22 of an instrument, as shown in FIG. 5. The LED light source 30 includes a substrate 16, a blue LED chip 10 mounted on the substrate, a frame 12 which is attached to the substrate 16 to surround the blue LED chip 10 and has on an inner surface thereof a mirrored reflection surface, and a sealing member 14 which contains a YAG phosphor and has a convex lens-shaped surface. The LED light source 30 is connected to a mother board 20 of the instrument by a solder 18. A semi-transparent window 40 is provided in the casing 22 of the instrument to face a front surface of the LED light source 30. The window 40 has a light transmission rate held down to within 20 to 50% such that yellow of the sealing member 14 is not visible.
Exit light 42 emitted from the LED light source 30 is absorbed in and reflected on the semi-transparent window 40, and only 20 to 50% of the exit light is emitted to the outside of the instrument. In addition, because the exit light is scattered by particles which make the window semi-transparent, as shown by dashed lines, the light is scattered through a range wider than an ideal range of about ±30° to an optical axis. This results in a low efficiency of light illuminating the subject.
It should be noted that there has also been proposed a technology to resolve a problem that light emitted from the blue LED chip 10 is also scattered by the YAG particles in the sealing member 14. By setting an angle of the mirrored reflection inner surface of the frame 12, the scattered light can be reflected in a predetermined direction. Also, by disposing the sealing member 14 in the frame 12, the sealing member 14 does not project from the frame 12 (see Patent Document 3).
A similar technology to this is disclosed in Patent Document 4. The technology has a structure in which a half-mirror processing is applied to a front window of an LED light source, and the window is disposed adjacent to an image pickup device so that a person who is a subject can know the orientation of a camera by way of checking an image reflected on the window. The fact that “a supplementary light source disposed opposite to the subject on a half-mirror surface is not visible from the outside” is disclosed in paragraph 0007 of Patent Document 4.
However, when the half mirror is formed by an almost completely transparent plate with a light transmission rate of 90%, the subject is not reflected on the window, and for the person as the subject to recognize an image of the subject on the half mirror, it is necessary to set the light transmission rate to be 50% or less. This may results in a large loss of light at the window, so that there is no significant difference between the technology proposed in Patent Document 4 and the conventional art mentioned in FIG. 5 regarding illumination efficiency. The increased costs of forming the half-mirror must also be taken into account.    [Patent Document 1]
Japanese Patent Application Publication No. H5-152609    [Patent Document 2]
WO98/05078    [Patent Document 3]
Japanese Patent No. 2998696    [Patent Document 4]
Japanese Patent Application publication No. 2003-287783